This invention is a process for the separation of gases. More particularly, the instant invention concerns a liquid membrane process for separating gases.
Gas separation is a large portion of the cost of natural gas processing and of numerous enhanced oil recovery schemes. To begin with, ethane, propane, and higher homologues must be removed from natural gas to make a saleable methane product. The higher alkanes can then be used for many purposes. Other separations may be needed to further separate the higher alkanes.
Gas separation is also a large part of the cost of numerous enhanced oil recovery schemes. Miscible floods employing carbon dioxide or intermediate hydrocarbons usually require the separation and reinjection of produced miscible agent to be economical.
Most of the cost of gas processing is related to gas separation. Cryogenic separation requires expensive compression and refrigeration. Absorption processes require costly solvent reconditioning. Membrane technology is being investigated to avoid much of the energy intensive operations as well as reduce the amount of capital investment and operating costs.
Polymer membranes are being used commercially in many processes, the most famous being Monsanto's Prism process. Polymer membranes have the drawback that the mass flow through the membrane per unit area is very small. This means that the surface area of the system has to be very large. The Prism process uses bundles of small diameter tubes in the configuration of a heat exchanger with one end blocked off. This gives the separator a large surface area, but the unit still needs to be fairly large to achieve suitable flow rates.
Liquid membranes achieve much larger mass flow rates per unit area due to their higher permeabilities. Second, they may also have separating factors higher than polymer membranes. Third, polymer membranes have the disadvantage of preferential transport of lower molecular weight species, which for many separation processes comprise the majority of gas volume. On the other hand, liquid membranes preferentially transport gases having lower equilibrium K values, which for separations such as alkanes are the lower volume, heavier alkanes.
Most known liquid membranes have been aqueous solutions with a carrier complex as the active agent. In such a process, a compound goes through a reversible reaction at both the upstream and downstream sides of the membrane. Such membranes are poor for hydrocarbon transfer, but perform well with acid gases such as carbon dioxide or hydrogen sulfide.
The use of hydrocarbon liquid membranes to separate low molecular weight hydrocarbon gases has been disclosed in Brennan, M. S., Fane, A. G., Fell, C.J.D., "Natural Gas Separation Using Supported Liquid Membranes," AIChE Journal, Vol. 32, No. 1, p. 558 (1986); and Dobitz, John K., "Experiments In Hydrocarbon Gas Separation Using Liquid Hydrocarbon Membranes," presented at the American Institute of Chemical Engineers Fall Meeting, Nov. 15-18, 1987, New York.